1. Field of the Invention
This invention relates to a method for the production of alkali metal phosphates and phosphoric acid by the acidulation of phosphate rock and more particularly to a method for the acidulation of phosphate rock in the substantial absence of fluorine evolution, recovery of the fluorides as useful solid values, and recovery of fluoride-free products.
2. Description of the Prior Art
Phosphoric acid plants are currently operated utilizing a basic and well known process for the acidulation of phosphate rock which comprises reaction of the rock with sulfuric acid to form phosphoric acid with subsequent reaction of the phosphoric acid, with for example ammonia to produce monoammonium phosphate (MAP) and diammonium phosphate (DAP). The phosphoric acid formed in this process is called wet process phosphoric acid. In this reaction, a by-product is gypsum having the chemical formula CaSO.sub.4 .multidot.2H.sub.2 O. Essentially all phosphate rock contains some fluorine, normally in the 3.0 to 4.0% range, and the acidulation reaction usually generates gaseous fluorides.
In recent years, both air and water pollution laws and regulations have become more stringent and are now being enforced more rigorously. Operating companies have had many pollution problems with fluorine emission into the atmosphere and with the by-product gypsum from these phosphoric acid plants. Thus, an important problem in the operation of these wet process phosphoric acid plants has been in the expensive methods for handling the large amounts of fluorine compounds which are liberated in the gaseous and aqueous effluents from such plants. In some phosphate complexes from 10,000 to 30,000 tons per year of fluorine compounds may be liberated by various methods. It is estimated that in a typical wet process phosphoric acid plant, a portion of the fluorides are evolved into the atmosphere in gaseous form, such as hydrogen fluoride and silicon tetrafluoride, which can destroy vegetation and affect other facilities in close proximity to the plant if they are not scrubbed out and such scrubbing systems are not always effective. A second portion of the fluorine is found in the gypsum dumps and is subject to leaching into groundwater and streams. Still another portion of the fluorine remains with the final products and when such products are used as when adding fertilizers to the soil, they change the soluble and insoluble fluoride levels. It is only in recent years that studies have been made on the effects of fluorides contained in the final product and indications seen clear that they may have a deleterious effect on the long range producing ability of the soil. See for example Kudzin et al., Chem. Ab., 73, 870534 (1970).
In these systems, any conventional wet process phosphoric acid technology accomplishes two primary objectives, namely: (1) phosphate rock acidulation, and (2) the growth of readily filterable calcium sulfate crystals either as the dihydrate (gysum), or as the hemihydrate. Conventional phosphoric acid technology carries out both of these objectives essentially simultaneously which leads to a number of environmental and purification problems almost immediately. The presence of strong sulfuric acid in the acidulation phase releases fluorides as HF, SiF.sub.4, and/or H.sub.2 SiF.sub.6. This poses a serious fluoride containment and subsequent recovery problems. Furthermore, unless excess sulfate levels are carefully and closely controlled, minute gypsum crystals can and will blind rock particles and usually result in poor P.sub.2 O.sub.5 recovery. The presence of free H.sub.2 SiF.sub.6 in the acid system leads to severe scaling and excessive maintenance costs even with improved design features to minimize this effect.
There is a great deal of literature and patent art related to attempts to remove the fluoride values from fluorine-containing phosphate rock in the operation of a phosphoric acid plant including methods for suppressing the evolution of fluoride values in the operation of a process and/or attempting to scrub the fluorine from effluent gases and waste water. Two such methods are described in U.S. Pat. Nos. 2,954,275 and 2,976,141 to Carothers et al, which use sodium or potassium compounds to suppress the fluorides so that they are concentrated in the gypsum cake. These patents indicate that this is achieved by adding a suppressing amount of an alkali metal salt to the acidulation reaction. However, these processes were conducted in the presence of sulfuric acid in the acidulation reactor and the process had incomplete control on fluoride decomposition and evolution during acidulation.
Other prior art patents have been noted which have also attempted to overcome the problem of fluorine evolution and the reduction of the amount of fluorine contained in final products. A reference of this type is British Pat. No. 735,086 (1955), which discloses the acidulation of phosphate rock by a two step procedure using a strong mineral acid such as nitric acid or hydrochloric acid. According to this patent, an initial low temperature acidulation at 20.degree.-50.degree. C. is carried out with the addition of an alkali, for example, ammonia or lime, as a precipitating agent in a quantity sufficient to precipitate substantially the whole of the fluorine and other impurities but insufficient to precipitate a substantial amount of calcium phosphates. After precipitation and optional removal of this precipitate, indicated as being calcium fluoride by the patentee, the mixture is heated to 75.degree. C. and the acidulation completed by adding a further quantity of alkaline precipitating agent. This. patent, however, does not suggest that the use of the strong mineral acid in even the low temperature procedure is going to necessarily provide sufficient reaction to result in at least some fluorine evolution. Moreover, it is doubtful that the precipitate recovered will be in the form of a calcium fluoride. Therefore, while this patentee appreciated the concept of attempting to remove the fluorides prior to the completion of the acidulation reaction, it is clear from a reading of this patent that he did not accomplish the intended purpose.
In a similar process, in U.S. Pat. No. 3,431,096 to Hill et al, a process is disclosed for reducing evolution of fluorine values in formation of triple superphosphate fertilizer by reaction of phosphate rock and phosphoric acid wherein ammonia or urea is added to suppress the fluorine evolution. However, in this patent, there is no provision for removal of the fluorine values from the product and therefore even if the fluorine evolution is prevented, the fluorine values will be retained in the resulting product and therefore distributed to the soil when it is used as a fertilizer.
In a series of patents issuing from the mid 1940's to early 1960's there are disclosed processes for the defluorination of phosphate rock and the production of defluorinated calcium phosphates. In these U.S. Nos. 2,337,498; 2,442,969; 2,893,834; and 2,997,367, the defluorination reaction is carried out by subjecting a mixture of phosphate rock, phosphoric acid and an alkali metal material to calcination, that is by reaction at temperatures as high as 1000.degree. C. to 2200.degree. C. Obviously, under these conditions the fluorine is going to be rapidly evolved or if not evolved certainly will remain in the final product, said to be an animal feed. Therefore, these patents did not provide a solution to the problem of fluorine evolution and of fluorine remaining in the final product.
Two additional patents of pertinence to processes of this type are U.S. Pat. Nos. 2,567,227 and 2,728,635 to Miller which disclose the acidulation of phosphate rock with phosphoric acid to form monocalcium phosphate, cooling to solidify the monocalcium phosphate and then converting it to dicalcium phosphate by hydrolysis. In the earlier patent, it is indicated that the fluorine in the rock is vaporized in the system, circulates throughout the system and/or leaves the system with the calcium phosphate. The later patent indicates that the process of U.S. Pat. No. 2,567,227 provided a final calcium phosphate product having a fluorine content too high to be of animal feed grade. The solution to this problem in the later patent was the addition of some dilute sulfuric acid in the acidulation step which would of course lead to additional fluorine evolution during the first step. There are also patents known in the art which indicate that it is known to acidulate phosphate rock with phosphoric acid and to then recover solid monocalcium phosphate by cooling of the resulting solution and recovering the monocalcium phosphate. Processes of this type are disclosed for example in U.S. Pat. Nos. 3,494,735 and 3,645,676. In addition, U.S. Pat. Nos. 3,619,136 and 3,792,151 to Case disclose the reaction of phosphate rock with recycle phosphoric acid at temperatures of about 125.degree.-180.degree. F. (52.degree. C. to 83.degree. C.) to form a solution of monocalcium phosphate, reacting the latter solution with sulfuric acid to produce phosphoric acid and calcium sulfate, precipitating the calcium sulfate, and recycling a portion of the phosphoric acid to the phosphate rock reaction. These patents point out that under the conditions recited, fluorides are not evolved but remain primarily unreacted and may be found with insoluble materials although a portion remains in the phosphoric acid solution product. Thus the products are going to be contaminated with fluorides. It is also known to react phosphate rock or a solubilized form with sulfuric acid and KHSO.sub.4 in combination with other steps and this reaction is described in U.S. Nos. 3,697,246 and 3,718,253. None of these patents disclose the unique combination of steps and advantages achieved by the process of this invention.
Other patents in the phosphoric acid and fluoride art suggest methods for use of scrubbing and recycling plants in an effort to contain or convert the fluorides evolved so that as much as possible of the fluorine can be recovered. Nevertheless, in all of these earlier approaches to the problem, provisions are never made for disposing of the approximately one-tenth or more of fluoride contained in the final product nor are there provisions made for converting the fluorides to useful products.
German Offenlegungschrift No. 2,257,948 disclosed the reaction of K.sub.2 SiF.sub.6 with KOH and lime to ultimately form calcium fluoride. However, the K.sub.2 SIF.sub.6 is not obtained from the acidulation of phosphate rock.
It is to be appreciated therefore, that the process of the invention provides an improvement over the processes of the references discussed herein and all other references of which applicant is aware. The present invention provides a system which substantially eliminates the problem of the fluorine evolution in the acidulation of fluorine-containing phosphate rock with phosphoric acid, the recovery of useful products from the acidulation reaction and the recovery of the fluorine contained in the rock in a usable form. Therefore, the present invention provides a unique combination of steps and advantages not appreciated heretofore in the prior art as none of these prior references disclose the unique combination of steps and results of this invention.